Silica-reinforced Rubber Compounds Research Articles

Saccharide-containing conjugates as eco-friendly coupling agents for silica reinforced rubber compounds

Silica reinforced rubber is used as a tire tread compound due to its desirable viscoelastic properties. To achieve proper silica dispersion in rubber compound, silane-based coupling agents are required. In this study, we synthesize saccharide-containing molecules and investigate their uses as green coupling agents for the development of environmentally friendly tires. Two saccharide conjugates are prepared: maltose-polybutadiene (Mal-PB) by coupling reaction between alkyne-functionalized Mal and azide-functionalized PB; and maltose-cystamine-maltose (Mal-Cys-Mal) by reductive amination of the reducing end of Mal. Both conjugates are used in place of bis(triethoxysilylpropyl)disulfide (TESPD) for rubber compounding. We find that Mal-PB serves as a good compatibilizer for silica/rubber interface but cannot bridge silica fillers to rubber chains due to lack of sulfur element. Mal-Cys-Mal shows better potential as a coupling agent. The rubber compound prepared with Mal-Cys-Mal shows increased bound rubber content and improved silica dispersion, and the resulting vulcanizate shows modestly improved viscoelastic properties and much improved tensile properties. Overall both saccharide conjugates reported can effectively enhance silica dispersion in rubber compounds, and the sulfur-containing Mal-Cys-Mal has the potential to replace TESPD as a greener coupling agent for tire tread formulation.

On the various roles of 1,3-DIPHENYL Guanidine in silica/silane reinforced sbr/br blends

The aim of this study was to evaluate the various roles of 1,3-DiPhenyl Guanidine (DPG) in silica-reinforced rubber compounds. Two roles of DPG are well known to be: adsorption onto silica surface to reduce the acidic sites and second to boost the silanization reaction as secondary accelerator. However, these two roles are in a way conflicting. When DPG molecules occupy the reactive silanol sites on silica by adsorption, then the access of silane molecules needed for silica-rubber coupling is blocked. Therefore, it may be assumed that there is another role DPG plays in silica filled rubber compounds. In order to evaluate another role which can reconcile these contradictory functions of DPG, a series of rubber compound mixings was performed with different DPG concentrations in the masterbatch and final stages. Additionally, a series of model reaction experiments has been done in combination with other ingredients, such as zinc oxide and stearic acid. According to the results a new role of DPG is observed: DPG possibly reacts with the silane coupling agent bis-(triethoxysilylpropyl)tetrasulfide (TESPT) and releases sulfur during the mixing process, which enhances filler-polymer coupling, but reduces cure rate and crosslink density.

Enhancing the Silanization Reaction of the Silica-Silane System by Different Amines in Model and Practical Silica-Filled Natural Rubber Compounds.

Diphenyl guanidine (DPG) is an essential ingredient in silica-reinforced rubber compounds for low rolling resistance tires, as it not only acts as a secondary accelerator, but also as a catalyst for the silanization reaction. However, because of concern over the toxicity of DPG that liberates aniline during high-temperature processing, safe alternatives are required. The present work studies several amines as potential alternatives for DPG. Different amines (i.e., hexylamine, decylamine, octadecylamine, cyclohexylamine, dicyclohexylamine, and quinuclidine) are investigated in a model system, as well as in a practical rubber compound by taking the ones with DPG and without amine as references. The kinetics of the silanization reaction of the silica/silane mixtures are evaluated using model compounds. The mixtures with amines show up to 3.7 times higher rate constants of the primary silanization reaction compared to the compound without amine. Linear aliphatic amines promote the rate constant of the primary silanization reaction to a greater extent compared to amines with a cyclic structure. The amines with short-alkyl chains that provide better accessibility towards the silica surface, enhance the primary silanization reaction more than the ones with long-alkyl chains. The different amines have no significant influence on the rate constant of the secondary silanization reaction. The amine types that give a higher primary silanization reaction rate constant show a lower flocculation rate in the practical compounds. For the systems with a bit lower primary silanization reaction rate, but higher extent of shielding or physical adsorption that still promotes higher interfacial compatibility between the elastomer and the filler surface, the rubber compounds show a lower Payne effect which would indicate lower filler-filler interaction. However, the flocculation rate constant remained high.

REINFORCEMENT OF NATURAL RUBBER BY SILICA/SILANE IN DEPENDENCE OF DIFFERENT AMINE TYPES

ABSTRACT Diphenyl guanidine (DPG) is the most commonly used secondary accelerator in silica-reinforced rubber compounds because of its additional positive effect on the silanization reaction and deactivation of free silanol groups that are left over after the silanization. However, because of health and safety concerns about the use of DPG, which decomposes to give highly toxic aniline during high processing temperature, safe alternatives are required. This work investigates the effect of various types of aliphatic amines having alkyl or cyclic structures and similar pKa (i.e., hexylamine [HEX], decylamine [DEC], octadecylamine [OCT], cyclohexylamine [CYC], dicyclohexylamine [DIC], and quinuclidine [QUI]) on the properties of silica-reinforced natural rubber (NR) compounds by taking the ones with DPG and without amine as references. When compared with the compound without amine, the use of all amine types reduces filler–filler interaction (i.e., the Payne effect) and enhances filler–rubber interaction, as indicated by bound rubber content and decreased heat capacity increment. The amines with alkyl chains can reduce the Payne effect and enhance cure rate to a greater extent compared with the amines with cyclic rings as a result of better accessibility toward the silica surface and a shielding effect because of less steric hindrance. The longer carbon tails on linear aliphatic amines ranging from HEX, DEC, to OCT lead to a lower Payne effect, lower heat capacity increment, higher bound rubber content, and higher modulus as well as tensile strength. Overall, the use of OCT provides silica-reinforced NR compounds with properties closest to the reference one with DPG and can act as a potential alternative for DPG.

The Development of Sulphur-Functional Silanes as Coupling Agents in Silica-Reinforced Rubber Compounds. Their Historical Development over Several Decades

Where silica is used as an active filler in rubber vulcanisates, a bifunctional silane has to be added to achieve the full reinforcement potential in almost all cases. Sulphur vulcanisation is the crosslinking method of choice in many fields of application, particularly because of the outstanding dynamic properties of the resulting parts. Since sulphur-functional silanes are able to participate directly in sulphur vulcanisation, this substance class has become the preferred coupling agent for the sulphur-based vulcanisation of silica-filled rubber compounds. This paper outlines in four stages the history of sulphur-silane development over the last few decades.

FLOCCULATION, REINFORCEMENT, AND GLASS TRANSITION EFFECTS IN SILICA-FILLED STYRENE-BUTADIENE RUBBER

Abstract The introduction of silanes to improve processability and properties of silica-reinforced rubber compounds is critical to the successful commercial use of silica as a filler in tires and other applications. The use of silanes to promote polymer–filler interactions is expected to limit the development of a percolated filler network and may also affect the mobility of polymer chains near the particles. Styrene-butadiene rubber (SBR) was reinforced with silica particles at a filler volume fraction of 0.19, and various levels of filler–filler shielding agent (n-octyltriethoxysilane) and polymer–filler coupling agent (3-mercaptopropyltrimethoxysilane) were incorporated. Both types of silane inhibited the filler flocculation process during annealing the uncured rubber materials, thus reducing the magnitude of the Payne effect. In contrast to the significant reinforcement effects noted in the strain-dependent shear modulus, the bulk modulus from hydrostatic compression was largely unaltered by the silanes. Addition of polymer–filler linkages using the coupling agent yielded bound rubber values up to 71%; however, this bound rubber exhibited glass transition behavior which was similar to the bulk SBR response, as determined by calorimetry and viscoelastic testing. Modifying the polymer–filler interface had a strong effect on the nature of the filler network, but it had very little influence on the segmental dynamics of polymer chains proximate to filler particles.

Flocculation in Silica Reinforced Rubber Compounds

Abstract Flocculation plays an important role in reinforcement of silica filled rubber compounds, even if coupling agents are applied. It is well known that silica tends to flocculate during the early stages of vulcanization, when no dense rubber network has been formed yet. In the present study, flocculation was monitored by following the change in storage modulus at low strain, the so-called Payne effect, using a RPA2000 dynamic mechanical tester. The kinetic parameters: the rate constant and the activation energy of the silica flocculation were calculated according to the well-known Arrhenius equation. On basis of the value of the activation energy obtained for flocculation, it can be concluded that the silica flocculation is a purely physical phenomenon. Bound rubber measurements were also done in order to estimate the interfacial interaction layer between silica and polymer resulting from the coupling agent. The silica flocculation rate decreases with increasing interfacial interaction layer on the silica surface. This indicates that the decrease of the flocculation rate is due to the shielding effect of the coupling agent. It is argued that the attractive flux from forces related to polarity differences between the silica and the rubber is the determining factor for silica flocculation.

Mechanistic aspects of the role of coupling agents in silica–rubber composites

Compared to carbon black, the use of silica as reinforcing filler for rubber results in lower hysteretic losses, for tyre applications leading to lower rolling resistance and consequently fuel savings. The compatibility of hydrophilic silica with a hydrophobic rubber polymer matrix is generally poor. Adding bi-functional coupling agents to the compounds, commonly bis(triethoxysilylpropyl) tetrasulphide (TESPT), enhances filler-matrix compatibility. The degree of hydrophobation of silica during rubber mixing then depends on many mutually interacting factors. Irreproducible conditions during mixing and vulcanisation are major causes of irreproducibility of silica-reinforced rubber compounds. Depending on the chemical composition of the coupling agent, the ultimate temperature obtained during the mixing process turns out to be the main factor governing the reactions of the coupling agent: the formation of a proper bond between the silica and the coupling agent, while avoiding a premature reaction with the rubber polymers, leading to premature scorch during mixing. Further, the mechanistic aspects of the reaction of various coupling agents, variants on TESPT, are covered, with silica as well as with the rubber. Of great importance are: the carbon and sulphur chain-lengths within the coupling agents, whether corrections are applied in the compound with elemental sulphur relative to the sulphur contained in the reference TESPT, and the moments the correcting amounts of sulphur are added to the compounds: during the first mixing stage, or together with the curing ingredients later-on in the process. The tensile properties of the vulcanised compounds are most prominently influenced. This is indicative of the dual role of sulphur: on the one hand as direct curative, on the other hand as part of the coupling agent becoming attached to the rubber polymers.